I try to treat my teen as though he were already an adult (note: not that he IS one, just that I try to treat him like one when it comes to how I interact with him).
If, say, I was standing there interrupting you, how would you react? Would you snarl at me, or would you say "I'm right in the middle of this, please give me a few minutes to wrap it up"?
At least if I'm using the same words as I would towards an adult, I can be fairly certain that I'm not going to be critical. Just yesterday, I had to ask him to stop following me around like a lost puppy. Of course I didn't word it that way, I said "If you don't mind, I'd like a couple of minutes to be alone with my thoughts." He let me have a couple of minutes, so far as I know he didn't feel criticized, and he learned a useful phrase (actually he's heard that phrase from me enough that I've seen him use it with others).
Preparing your kids for how to set their own boundaries? Priceless. Getting your own needs met in the process? Heaven!